Scanning probe microscopes (hereinafter, "SPM's") are instruments which provide a microscopic analysis of the topographical features or other characteristics of a sample surface by causing a probe to scan the sample surface, typically in a raster pattern.
One type of SPM is a "contact-type" SPM in which the probe consists of a cantilever with a sharp tip located near its free end. The other end of the cantilever is coupled to the body of the SPM. As the cantilever is scanned, the tip moves relative to and interacts with the sample surface. A change in the tip elevation due to interaction with height deviations on the sample surface causes the deflection of the cantilever to change. The tip elevation is detected and plotted on a graphical user interface device such as a computer monitor.
There are several ways of determining the change in tip elevation. One is by first determining the degree of deflection variance of the cantilever. This degree of deflection variance can be determined by pointing a laser beam towards a reflector positioned on the cantilever. An image detector receives the reflected laser beam. As the deflection of the cantilever changes, the angle of incidence between the unreflected laser beam and the reflector on the cantilever will change. Thus, the change in deflection of the cantilever will cause the reflected laser beam to radiate on a portion of the image detector that is different than the portion of the image detector that would have received the laser beam without the change in deflection of the cantilever. The output of the image detector is used to determine the deflection of the cantilever because the geometries and materials (and thus deflection characteristics) of each portion of the cantilever is known. From this data, the change in tip elevation is determined and plotted on a graphical user interface for convenient user viewing.
In constant force scanning, as the tip elevation changes are detected, the end of the cantilever coupled to the body of the microscope is moved relative to the sample surface such that the cantilever has a constant deflection. The movement means is coupled in a feedback system such that the cantilever moves substantially simultaneously with the vertical elevation change of the tip. The end result is that the cantilever never has significant deflection variances. Once a minor deflection change is detected, the cantilever moves in response to counteract the deflection change. Therefore, the tip applies a constant force on the sample surface.
At the atomic scale, an effective actuator for causing such small movements is the piezoelectric tube. The rigid end of the cantilever and/or the sample surface is attached to a piezoelectric tube. The piezoelectric tube(s) respond to a signal by expanding or contracting. This feedback process occurs continuously such that the tip exerts a constant force on the sample surface. A more detailed discussion on contact mode SPM's may be found in U.S. Pat. No. 5,376,790 (Linker et al.) issued Dec. 27, 1994 entitled "Scanning Probe Microscope" which is incorporated herein by reference in its entirety.
An atomic force microscope (hereinafter, "AFM") is a type of SPM which is capable of detecting topographical features often down to the atomic scale. It is well known that the more detailed the resolution requirements, the sharper the tip of the probe must be. As the tip travels along and in contact with the surface, the tip will inevitably wear. AFM and other fine resolution SPM's are particularly sensitive to tip wear because the AFM's require atomic levels of resolution. Tip wear degrades the resolution ability of the probe. Therefore, the tip must be replaced often, especially in AFM's. Currently, tips are so small and delicate that it is the entire probe assembly that is replaced after the tip has worn so much that desired resolutions can no longer be obtained. Thus, it is the entire probe assembly which must be replaced often.
Wearing of the tip is inevitable in contact type SPM's. A tip can only cover a finite distance before becoming worn so much that desired resolution cannot be attained. In AFM's, this finite distance is relatively small because the desired resolution is so detailed. Currently, a contact mode SPM tip will last approximately a few hours before reaching this finite distance. However, as finer details are required in response to the rapid down-scaling of feature dimensions on integrated circuits, the finite distance will decrease. Furthermore, technology is enabling faster scanning speeds. One such technology is disclosed in pending U.S. application Ser. No. 08/558,563, (Minne et al.), now U.S. Pat. No. 5,883,705, which is incorporated herein by reference in its entirety. As higher speed scanning is developed, the finite distance that wears the tip to intolerable levels will be reached more rapidly, thereby decreasing the lifetime of the tip. As changing of the cantilever becomes more frequent, operators have a motivation to settle for more degraded resolution requirements, thereby compromising the resolution of the scanning. This compromise is unnecessary in light of the principles of the present invention.
Furthermore, a cantilever capable of high speed imaging is much more complex and expensive than a regular cantilever. Thus, as scanning is performed at higher speeds, the user will have to replace more expensive cantilevers even more often. This is expensive and provides another motivation for the user to settle for degraded resolution. Therefore, what is desired is an SPM which has probes capable of traveling larger distances in contact with sample surfaces before requiring probe replacement without sacrificing resolution.